In general, optical sensor arrangements are based on the principle that the radiation which is emitted by a radiation source passes through a measured distance, in which the analyte to be detected is present, and the change which occurs in the radiation intensity because of the presence of the analyte is evaluated by means of a corresponding detector. The radiation intensity can be affected by absorption, scattering or turbidity, and the analyte can be present in the liquid or gaseous phase.
Known gas sensor arrangements are used to detect the most diverse analytes, such as methane or carbon dioxide. They are based on the property of many polyatomic gases, that they absorb radiation, particularly in the infrared wavelength range. This absorption occurs at a wavelength which is characteristic of the relevant gas, for example for CO2 at 4.24 μm. Using infrared gas sensors, it is therefore possible to establish the presence of a gas analyte and/or the concentration of this gas analyte in a gas to be measured.
Known gas sensor arrangements, such as are disclosed, for instance, in DE 10 2004 028077 A1 or DE 10 2004 007 946 A1, have a radiation source, an absorption distance, for example, a measurement space, and a radiation detector. According to the known Lambert-Beer law, the radiation intensity which the radiation detector measures is a measurement of the concentration of the absorbing gas. In the case of these so-called NDIR (non dispersive infrared) sensors, a broadband radiation source is used, and the relevant wavelength can be set via an interference filter or grating.
Carbon dioxide detection in particular is gaining increasing significance today in many application areas. For instance, the quality of the internal air can be monitored, the cleaning cycle of self-cleaning ovens can be monitored, and the supply of CO2 to plants in greenhouses can be regulated. In the medical field, for example in anesthesia, the air of a patient's breath can be monitored, and finally, wherever the danger of escaping CO2 exists, for instance in correspondingly filled air-conditioning systems, a carbon dioxide sensor can be used in a warning system.
In the motor vehicle field, carbon dioxide detection can be used to increase the energy efficiency of heating and air-conditioning, to monitor the CO2 content of the interior air, to cause a fresh air supply by controlling an appropriate fan flap only if necessary, for example, in the case of increased CO2 concentration.
Also, modern motor vehicle air-conditioning systems are based on CO2 as the coolant, so that CO2 gas sensors can monitor escaping CO2 to detect leaks/defects. Particularly in the motor vehicle field, such gas sensor arrangements must meet the highest requirements for robustness, reliability, miniaturizability, and long life cycles.
In the case of known gas sensor arrangements, such as arrangements which are disclosed in DE 10 2004 028 077 A1, the radiation source is not usually operated uniformly, but pulsed at a specified frequency. For example, the radiation source, which is usually in the form of a miniature incandescent lamp, is operated by direct current (DC) which is switched by rectangular pulses.
It has been shown that the lamps which are switched on and off, again and again, are exposed to increased stress because of the making current which occurs each time, and thus have a shorter lifetime. To extend the lifetime of such incandescent lamps, which are switched on in pulsed operation, and in particular to reduce the high making current peaks, a known method is to switch the lamp on by a semiconductor, and to provide an RC element in the control circuit, to make it possible to switch the lamp on smoothly.
Such a known radiation source is shown schematically in FIG. 1. However, at the time of switching on there is always an overcurrent at a multiple, usually five to ten times, of the nominal current. This is shown in FIG. 3, in which the lamp current upon switching on is shown over time, as curve 301.
The cause of this high peak current can be seen in that the incandescent lamp filament in the cold operating state of the lamp has a significantly lower electrical resistance than in the hot operating state. It is only after a few milliseconds of operation that the incandescent filament reaches its operating temperature and thus its final electrical resistance, and the current through the lamp approaches the operating current value.
It is therefore desirable to reduce the stress on a radiation source for an optical sensor arrangement in pulsed operation, and thus to increase its lifetime, without excessively increasing the size and production costs.